The purpose of this study to find out how safely we can add extra nutrients to human milk at different feeding volume.

Very low birth weight babies (weight less than three pounds and three ounces) are extremely fragile and lacking important nutrition due to premature birth. They need enough calories for catch-up growth. Giving adequate nutrition is very important for their growth and development. Human milk is the best food, however it is not enough to provide all the required calories and nutrients for catch up growth and maintain adequate strength of the bones (bone mineralization). For premature babies, extra nutrients (human milk fortifier) are usually added to the human milk to provide adequate nutrition. These extra nutrients will be derived from cow's milk. Currently, a practice of fortifying human milk varies. There is no clear information when to start adding extra nutrients to human milk.

Inclusion Criteria:

• Very low birth weight infants who receive human milk (mother's milk or banked donor human milk).

Exclusion Criteria:

• Major congenital anomalies such as cardiac defect, neural tube defect, chromosomal abnormality and gastrointestinal defect.
• Died or extended to die within 48 hours of life